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Natural Bridge
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Stop 14 Natural Bridge
Location: Death Valley National Park, CA.
Date: March 9, 2007

Natural Bridge Canyon is located in the cliffs of the eastern side of Death Valley and was carved in an alluvial fan of the Furnace Creek Formation which was deposited 3 to 5 million years ago during the late-Pliocene epoch.  While it took thousands of years for the canyon to be carved and sculpted by the erosive force of water, the actual removal of rock was accomplished over several short periods of flash-flooding.  Natural Bridge Canyon begins as a wide wash and becomes progressively narrower heading east to its head at dry falls.  Numerous geomorphic processes and features are visible within the canyon walls, revealing the complex and dynamic nature of the landscape.

 


View looking eastward down the canyon

 

Natural Bridge Canyon is named for the large natural bridge that spans the width of the canyon.  As periodic flash-flooding was eroding a channel into the ancient alluvial fan, the course of the channel changed at the current site of the natural bridge, bending slightly to the north (the abandoned channel can be seen on the north side of the canyon).  Erosion continued along the straighter stream channel, and as it undercut more rock, the channel deepened, forming the natural bridge that is seen today.

 


Diagram showing the formation of the large natural bridge within the canyon

 


View of the natural bridge from the west

 

Several smoothly rounded vertical chutes can be seen in the rock walls of the canyon (on both the north and south sides), and are especially well developed near the natural bridge.  Prior to extensive deepening of the canyon, small tributaries flowed into the main stream channel that carved the canyon.  However, due to the greater erosive force of the water flowing through the main channel, this larger stream deepened more rapidly than the smaller tributaries.  As a result, tributary channels were left hanging.  Once water from the tributaries reached the side of natural bridge canyon, it would cascade down the canyon walls in order to reach the main stream channel.  Over time, vertical chutes were created in the walls from the flowing water.

 


Vertical chute found in the canyon walls

 

The canyon walls also feature many wax drippings, or deposits of dissolved sediments formed after rainfall.  As water drips down the canyons walls, it evaporates quickly, leaving behind a thin film of silts and clays, creating dark stains on the lighter rock.

 


Wax drippings, or deposits of dissolved sediments, are prevalent on the walls throughout Natural Bridge Canyon

 

Further evidence of how water has sculpted this canyon can be seen where the canyon walls become concave in some areas and bevel out in others.  These irregularities provide evidence of past water levels and episodes of different downcutting stages.  Concave portions of the canyon walls indicate areas where water velocity was higher, enabling it to scour out and erode the rock.  Convex areas correspond to locations where stream channel velocity was lower, permitting less erosive action.  Where the wall takes on a concave form, most often the opposite canyon wall will exhibit a convex form and vice versa.  Examining the shape of the canyon walls fosters a reconstruction of past stream channel patterns as they meandered down and eroded into the ancient alluvial fan, creating the canyon itself.

 


Evidence of former water levels and episodes of various downcutting stages can be seen in
alternating convex and concave portions of the canyon walls

 

Many faults are exposed within the walls of Natural Bridge Canyon, highlighting past tectonic activity which has also influenced the formation of the landscape.  An “offset” fault is visible at the mouth of the canyon on both the north and south walls.  Bedding layers do not line up, and the amount they are offset is an indication of the amount of fault movement.  A larger fault is visible a short distance beyond the natural bridge in the north wall of the canyon.  Here, the fault has resulted in the formation of a cave and is thus referred to as the cave fault.  Good exposure of a turtleback fault is found just below the dry falls at the head of Natural Bridge Canyon on the canyon’s south wall.  These complex faults derive their name from the turtle shell-like appearance of the rock surfaces they expose.

 


Cave fault seen in Natural Bridge Canyon

 

At one point, the canyon is partially blocked by a large boulder measuring approximately 4 m by 3 m.  The rock of this boulder does not coincide with the rock of the canyon walls where it is trapped, indicating it must have come from further up the canyon.  Its striking size again is a testament to the powerful forces at work in the canyon.


Large boulder partially obstructing the passageway near the head of the canyon.  Note the distinct
composition of this rock as compared to that seen in the canyon walls.

 

 

By Jennifer Mikolajczyk